1. Field of the Invention
This invention relates to a submerged entry nozzle for use in a continuous casting process, in particular to a submerged entry nozzle for use in a continuous casting process suitable for casting various types of steel such as high concentration oxygen-containing steel, high concentration Mn-containing steel, Ca-treated steel, stainless steel.
2. Description of the Related Art
During a process of continuously casting steel, a submerged entry nozzle is usually employed to introduce molten steel from a tundish into a mold.
FIG. 8 is a cross sectional view schematically showing a typical structure of a submerged entry nozzle made according to the prior art. As shown in FIG. 8, within the main body 1 of a conventional submerged entry nozzle there are formed a vertically arranged elongated internal hole 2 and a plurality of discharge openings 3 arranged exactly perpendicular or generally perpendicular to the internal hole 2. The molten steel from the tundish is at first introduced into the internal hole 2 and then caused to flow in different directions through the discharge openings 3, thereby allowing the molten steel to be injected uniformly into a mold.
Conventionally, one of the most widely used submerged entry nozzles has been an Al.sub.2 O.sub.3 --SiO.sub.2 --C (hereinafter, simply referred to as "AG") submerged entry nozzle.
FIG. 7 is another cross sectional view schematically indicating pattern for arranging different materials in a conventional AG submerged entry nozzle. As shown in FIG. 7, a mold powder line portion 14 of the nozzle is formed by a ZrO.sub.2 --C material, with other portions, i.e., the main body portions 11 of the submerged entry nozzle being formed of an AG material.
Although such an AG submerged entry nozzle has excellent resistance against spalling, when a submerged entry nozzle is used in a process for casting various types of steels such as high concentration oxygen-containing steel, high concentration Mn-containing steel, Ca-treated steel and stainless steel, abnormal melting loss will often occur. Due to such melting loss in an AG submerged entry nozzle, there will occur changes in the composition of the steel being cast, in particular undesirable increases in the carbon concentration of the cast product.
The above phenomenon will not only cause problems with shortened usable submerged entry nozzle life, but also troubles in the steel manufacturing process. For this reason, an extremely important technical task is to develop a new type submerged entry nozzle which does not produce the above problems.
On the other hand, in order to solve the above problems, improved submerged entry nozzles have been suggested whose internal surface is formed by a carbonless refractory material containing not over 5 wt % of SiO.sub.2 but containing 90 wt % or more of one or more substances selected from the group consisting of Al.sub.2 O.sub.3, MgO, ZrO.sub.2 (Japanese Patent Laid-Open No. 3-243258).
In order to solve the above problems, the inventors of the present invention have carried out a research on the mechanism of melting loss in an AG submerged entry nozzle under conditions where it is used for casting various types of steel such as high concentration oxygen-containing steel, high concentration Mn-containing steel, Ca-treated steel, stainless steel. The mechanism which the inventors have found will be discussed in the following.
Namely, when a refractory material is in contact with molten steel, graphite on the working surface will be quickly dissolved into the molten steel, causing a phenomenon which may be represented by the following equation: EQU C(s)=C (1)
As a result, the working surface will become just Al.sub.2 O.sub.3 --SiO.sub.2 oxides.
After that, when the steel to be cast is high concentration oxygen-containing steel, high concentration Mn-containing steel or stainless steel molten, Mn and O and Fe in the molten steel will penetrate, in the form of MnO and FeO, into the working surface, causing a phenomenon which may be represented in the following equations. EQU Mn+O=(MnO) (2) EQU Fe+O=(FeO) (3)
Further, MnO--FeO type inclusion substances will impinge and adhere to the working surface.
The penetrated MnO and FeO penetrated from the above two sorts of phenomenon will react with the Al.sub.2 O.sub.3 and SiO.sub.2 on the working surface, thereby forming a liquid Al.sub.2 O.sub.3 --SiO.sub.2 --MnO--FeO type slag.
Since such a liquid slag is likely to flow away with the flow of the molten steel, the refractory material forming the submerged entry nozzle will suffer from a problem called melting loss.
Further, if the steel to be cast is Ca-treated steel, Ca will reduce Al.sub.2 O.sub.3 or SiO.sub.2, thereby forming CaO and thus causing a phenomenon which may be represented in the following equations: EQU SiO.sub.2 (s)+2Ca=2(CaO)+Si (4)
Al.sub.2 O.sub.3 (s)+3Ca=3(CaO)+2Al (5)
Such a CaO will penetrate the working surface.
Further, CaO--Al.sub.2 O.sub.3 type inclusion substances will impinge and adhere to the working surface.
As a result, on the working surface a liquid CaO--Al.sub.2 O.sub.3 --SiO.sub.2 type slag will be formed, so that the refractory material forming a submerged entry nozzle will also suffer from melting loss.
As may be clearly understood from the above discussion, the nozzle described in the above Japanese Patent Laid-Open No. 3-243258 encounters some problems which may be summerized as follows.
1) Even if a refractory material contains 90 wt % or more of Al.sub.2 O.sub.3, ZrO.sub.2, some undesired reactions represented by the above equations (2) to (5) will still occur, and some unwanted inclusion substances in a molten steel will similarly adhere to the working surface. As a result, it is impossible to avoid the formation of a liquid slag on the working surface and the melting loss of the refractory material forming the nozzle (such a melting loss is usually caused by such formation of liquid slag).
2) Further, in a case where a refractory material containing 90 wt % or more of Al.sub.2 O.sub.3, ZrO.sub.2 particularly MgO has been arranged to form the internal surface of the submerged entry nozzle including the discharge openings of the nozzle, it is likely that some cracks will occur in the vicinity of the discharge openings.
The reason for the above problem is that a refractory material containing 90 wt % or more of Al.sub.2 O.sub.3, ZrO.sub.2 or particularly MgO has a large coefficient of thermal expansion. Further, in the vicinity of the discharge openings of the submerged entry nozzle, there are many working surfaces that are subject to thermal shock, resulting in complex shapes where stress is likely to collect.
The inventors of the present invention, in accordance with the above findings and in view of the above problems, have suggested "a submerged entry nozzle having sufficient melting loss resistance and sufficient thermal shock resistance, which can be effectively used in a casting process suitable for casting various types of steel such as a high concentration oxygen-containing steel, high concentration Mn-containing steel, Ca-treated steel, stainless steel" (Japanese Patent Application No. 10-6143).